There has been a subscriber line interface circuit, in a digital method, that has a terminating resistor, a surge protection circuit, a power supply transformer, and transmission and reception circuit. In this surge protection circuit, which protects the subscriber line interface circuit from an external lightning surge, two protection elements (first protection element and a second protection element), in each of which a Zener diode is bi-directionally connected in series, are connected in series. In this structure, a third protection element and a fourth protection element are connected in series. The first protection element is connected to an end of a line and the fourth protection element is connected to the other end of the line. A connection point between the second protection element and the third protection element is grounded (see Japanese Laid-open Patent Publication No. 05-316641, for example).
There has been also a lightning surge protection circuit included in an integrated services digital network (ISDN) terminal apparatus in which a bi-directional constant-voltage diode is connected between two terminals on the primary side of a transformer. Two resistors are connected in series with one terminal on the primary side of the transformer, and other two resistors are connected in series with the other terminal on the primary side of the transformer. A first silicon surge protection element is connected between an earth circuit and a first connection point between the two resistors connected to the one terminal on the primary side of the transformer. A second silicon surge protection element is connected between the earth circuit and a second connection point between the other two resistors connected to the other terminal on the primary side of the transformer. A third silicon surge protection element is further connected between the first connection point and the second connection point. The structure, described above, of the lightning surge protection circuit is provided to protect a transformer and its secondary circuit from a lightning surge (see Japanese Laid-open Patent Publication No. 2002-077454, for example).